The ovarian function of mammalian females is regulated by the hypothalamus and pituitary, secreting gonadotropin releasing hormone (GnRH) and gonadotropins respectively. The gonadotropins are follicle stimulating hormone (FSH), which causes follicle maturation, and luteinising hormone (LH), which causes ovulation.
After each menses, the ovaries are stimulated by FSH released by the pituitary to grow a cohort of follicles. These follicles each comprise an oocyte (egg cell) which is enveloped by an orb of granulosa cells. During growth of the follicles several layers of granulosa cells are being formed. Follicle maturation during the normal menstrual cycle occurs in 12-14 days. Gradually, one follicle becomes dominant and the others become atretic. Maturation of the dominant follicle usually takes 5-7 days. As the number of granulosa cells increases more estrogen is secreted by these cells.
Once the dominant follicle has reached maturity, the follicle will burst (ovulate) under the action of a surge of LH which is released by the pituitary in response to the increased blood serum estrogen level (positive feedback). The oocyte is discharged from the follicle into the ampulla of the Fallopian tube, where fertilization may take place. The oocyte or embryo is transported to the uterus in 5-7 days, where implantation may occur in the midluteal phase.
The follicle that has discharged the oocyte is transformed into a new hormone producing organ, the corpus luteum. The corpus luteum produces amongst others progesterone and estrogens. The corpus luteum has a limited lifespan of about 12-14 days, unless pregnancy occurs. During the second part of that period, it ceases functioning, and as a result the blood level of estrogens and progesterone drops. The decline of progesterone causes shedding of the lining of the uterus and thus menstruation.
In particular in the area of ovulation induction, the past decades have shown the development and commercial introduction of numerous drugs assisting in fertility management of infertile couples. Amongst others, these include anti-estrogens (like clomiphene citrate and tamoxifen citrate), pulsatile GnRH, purified and recombinant gonadotropins, and GnRH agonists and antagonists. The specific drugs used and administration regimens chosen largely depend on the goal of the treatment, e.g. the induction of mono-ovulation in anovulatory females or the controlled ovarian hyperstimulation (COH) to induce multiple follicular development as an element in assisted reproductive technologies (ART). Examples of ART methods that are widely used to treat female and/or male factor infertility include intrauterine insemination (WI) and in vitro fertilization (IVF). IVF can be performed with and without intracytoplasmatic sperm injection (ICSI) and includes a subsequent embryo transfer step.
COH is nowadays widely used in ART. First results with COH were disappointing as a result of the occurrence of premature LH surges in at least 30% of the cases. Such a premature LH-surge may incite ovulation of oocytes and may frustrate harvesting of oocytes for in vitro fertilisation (IVF). It was found that the introduction of GnRH agonists allowed the prevention of premature LH surges as well as programmation of the treatment cycles. To date GnRH agonists are used in most of the cycles. However, GnRH agonists are peptides, requiring parenteral administration, are expensive and are not devoid of adverse effects (long treatment period, side effects, increased incidence of ovarian hyperstimulation syndrome, etc.).
Recently GnRH antagonists were introduced to prevent premature LH surges, to avoid the side effects related to the use of GnRH agonists and to simplify and shorten treatment protocols. However, there are concerns about the pregnancy rates observed with protocols using GnRH antagonists. Several studies have indicated that pregnancy rates for GnRH antagonists are lower than those achieved with GnRH agonists. Furthermore, like GnRH agonist, GnRH antagonists are peptides requiring parenteral administration, which is less favourable.
An area of key interest to IVF-researchers is the poor implantation rate of IVF embryos, responsible for the relatively low implantation rate per embryo. This has lead to the practice of multiple embryo transfers, which practice in turn has lead to high rates of multiple pregnancies. Such multiple pregnancies are considered a major drawback of ART.
As will be apparent from the above there is a need for a method of treating or preventing female infertility which does not employ the aforementioned GnRH analogues, but instead uses a substitute which is equally suitable for preventing premature endogenous LH surges, which produces equal or superior pregnancy rates, can be given orally, gives rise to less side-effects and/or is less expensive.